1. Field of the Invention
The invention relates to a process for the continuous production of a mat which is a preform for sheets, made from a mixture composed of fibers and/or pellets made from cellulose or wood and at least one hot-curing resin, and also to a preform and its use.
2. Description of the Related Art
U.S. Pat. No. 5,082,605 discloses a process for the production of a composite material which comprises a discontinuous phase of cellulose fibers, bonded and encapsulated within a continuous phase of a polymeric component, the polymeric component being composed mainly of polyethylene. The cellulose fibers have a ratio of length to diameter of from 2 to 12 and are mixed with the polymeric component, the temperature of the mixture being increased as far as the point of encapsulation. The encapsulated material is held within the encapsulation range while the particle size is reduced. The material is then extruded while its temperature is controlled within the encapsulation range. The fibers are essentially oriented in the direction of flow until the material reaches a heated die. Cellulose fibers and the polymeric material are mixed in a ratio of between about 40% by weight of polymer and 60% by weight of fibers and 60% by weight of polymer and 40% by weight of fibers, until the surface temperature of the mixture is between 143 and 177° C., or even higher. The surface temperature here depends on the polymeric material used. The mixture is then divided into portions whose maximum dimension is about 3.8 cm. These portions are then extruded, the temperature being held within the encapsulation temperature range. During the extrusion, the encapsulated fibers are essentially oriented in the direction of flow. The polymer is mainly a low-density polyethylene, but there may be a relatively small content of high-density polyethylene. There may also be a relatively small content of polypropylene in the polymer. Examples of products manufactured from the composite material are roofing shingles, floor tiles, panels, boards or frames for doors or for windows, and also items for the household or for gardens.
U.S. Pat. No. 4,228,116 describes formable sheets produced by continuous plastification and extrusion of an at least partially aggregated mixture made from 40 to 60% by weight of a thermoplastic resin, preferably polypropylene, and from 40 to 60% by weight of an organic, in particular cellulosic, filler material, such as wood particles. Before curing, the extruded material is rolled out to give a strip. This extruded, rolled-out strip is cut into sheets and these are used to manufacture, for example molded items, such as dashboards, instrument panels or other components for vehicles, or furniture or parts for furniture, or the like, by working the sheets at elevated temperature via pressing or other methods of forming, as are known in the prior art for the processing of thermoplastic materials. As an alternative, the sheets may be used for purposes for which sheets of timber or plywood are normally used.
WO 91/19754 discloses a mat impregnated with plastic and composed of a solvent-free plastic based on an epoxy resin and on a phenol novolak hardener, with an accelerator if desired. The proportion of volatile substances is below 0.5% by weight. To produce this mat, a solvent-free epoxy resin-phenol novolak mixture is heated to a temperature of from 60 to 140° C., at which the resin mixture has a viscosity of from 5 to 50 Pa.s. The preheated epoxy resin-phenol novolak mixture continuously permeates a web of fiber mat in a calender. The mat produced by this process has good storage stability and is suitable for producing hot-pressed, fully cured plastic parts with high resistance to chemicals and heat. An example of the fiber mat is a glass fiber mat.
It is a feature of the invention to provide a process which can extrude preforms with up to about 90% by weight of organic fiber content by conventional methods, without decomposition of the mixture. The feature extends to providing storage-stable preforms which can be used to mold fully cured sheets or sheet sections which have a substantially smooth or, where appropriate, a slightly structured surface.
This feature is achieved by means of a process which comprises:
(a) mixing from about 50 to about 90% by weight of fibers and/or pellets with from about 10 to about 50% by weight of hot-curing resin, where each of the percentages by weight is based on the total weight of the mixture,
(b) kneading and homogenizing the mixture made from fibers and/or pellets which break down to give fibers, and from at least one hot-curing resin, and passing the mixture onward to an extruder,
(c) heating the mixture in the extruder to an extruder exit temperature at which the resin begins to cure,
(d) extruding the mixture, which has not entirely cured, as a homogeneous composition and distributing the homogeneous composition within a calender line,
(e) exerting pressure onto the homogeneous composition in the calender line, to an extent which gives a mat which is a preform, and
(f) removing the preform and stabilizing its desired shape.
Another procedure which achieves the feature is that of:
(a′) mixing and kneading from about 50 to about 90% by weight of fibers and/or pellets which break down to give fibers, with from about 10 to about 50% by weight of hot-curing resin in an extruder,
(b′) varying extents of heating in the extruder of the mixture made from fibers and from at least one hot-curing resin,
(c) heating the mixture in the extruder to an extruder exit temperature at which the resin begins to cure,
(d) extruding the mixture, which has not entirely cured, as a homogeneous composition and distributing the homogeneous composition within a calender line,
(e) exerting pressure onto the homogeneous composition in the calender line, to an extent which gives a mat which is a preform, and
(f) removing the preform and stabilizing its desired shape.
The fibers used in the process are wood fibers whose moisture level is in the range from about 2 to about 50% by weight, in particular from about 10 to about 20% by weight. It is useful to knead the mixture in a mixer and then pass the same onward to an extruder from which the mixture is extruded without any back-pressure being exerted on the extruder screw. The hot-curing resins used in the process comprise thermosetting polymer compositions and/or resins with different extents of condensation, or are different types, or are mixtures of two or more of these resins. It is advantageous for the introduction of the resins to the mixer to be separate from that of the fibers and/or pellets. The extruder comprises zones with different temperatures, the temperature being controlled so that the temperature of the mixture is not more than about 120° C. at the exit from the extruder. The resin or thermosetting polymer composition preferably starts to cure or polymerize at this temperature which temperature however is preferably not high enough for full and entire polymerization or curing of the resin or of the mixture made from resin with fibers. In one embodiment of the process, the hot-curing resins have been selected from the group consisting of thermosetting polymers, phenolic resins, melamine resins, and epoxy resins.
The preform for hot-pressed, fully cured sheet sections or sheets made from a composite material made from fibers and from at least one hot-curing resin is composed of from about 50 to about 90% by weight of fibers and from about 10 to about 50% by weight of at least one hot-curing resin, the percentages by weight being based on the total weight of the composite material, where the resin of the preform has not entirely cured.
The preform is used as starting material for hot-pressed, fully cured sheet sections or sheets, by inserting one or more plies into a mold and fully curing these by hot-press molding.